This invention relates to a combinatorial weighing method for execution by a combinatorial weighing apparatus of the type which performs a combinatorial calculation to form combinations of weight values indicative of the weights of articles weighed by a plurality of weighing machines, selects a weight value combination having an optimum total weight with respect to a set weight, and discharges the articles corresponding to the selected combination.
Let us describe a weighing apparatus of the above kind in greater detail. The apparatus includes a plurality of weighing machines into which articles are introduced to be weighed, and a calculation control unit constituted by a microcomputer or the like, which is provided with the measured weight values from the weighing machines. The calculation control unit combines the weight values into combinations, calculates the total weight of each combination, compares each total weight value with a set target weight, selects the combination (referred to as the "optimum combination") the total weight value of which is equal to the set target weight or closest to the set target weight within upper and lower limit weight values, and causes those weight machines that have provided the weight values belonging to the optimum combination to discharge their articles.
FIG. 1 is a block diagram of an example of such a combinatorial weighing apparatus. The apparatus includes a plurality of weighing machines 1-1, 1-2, . . . 1-n each of which has a weight sensor (not shown) for weighing articles with which the weighing machines are supplied, the weighing machines producing weight data W1, W2, . . . Wn, respectively, indicative of the weights measured thereby. Each item of weight data is an analog value. It should be noted that each weighing machine also includes a weighing hopper (not shown) that actually receives the articles to be weighed, and a pool hopper (not shown) for resupplying the weighing hopper, as will be set forth below. The weight data W1, W2, . . . Wn are fed into a multiplexer 2, constituted by analog switches or the like, adapted to deliver the items of weight data one at a time in sequential fashion to an AD converter 3 in response to a changeover signal Sv received from a calculation control unit 4. The AD converter 3 converts the weight data W1, W2, . . . Wn received from the multiplexer 2 into digital values and feeds the digital weight data into the calculation control unit 4. The latter is composed of a microcomputer for performing a combinatorial calculation on the basis of the weight data and includes a processor, various memories (such as a program memory) and working memory, and input/output interface circuitry. An upper limit weight setting unit 5, a target weight setting unit 6, and a lower limit weight setting unit 7 respectively provide the calculation control unit 4 with an upper limit weight value Wp, a set target weight Wa, and a lower limit weight value Wo set thereby.
In operation, a packing machine (not shown) applies a timing signal St to the calculation control unit 4 when the packing machine is ready to pack a batch of weighed articles. The calculation control unit 4 responds by delivering the changeover signal Sv to the multiplexer 2. The latter responds in turn by sequentially supplying the AD converter 3 with the weight data W1, W2, . . . Wn indicative of the weights of the articles measured by the respective weighing machines 1-1, 1-2, . . . 1-n. The AD converter 3 converts the analog weight data input thereto into digital signals and feeds these signals into the calculation control unit 4. The latter, in response to a signal from the internal program memory, successively generates 2.sup.n -1 combination patterns (where n is the number of weighing machines) on the basis of the weight data from the AD converter 3, each patterns being a combination of weight values, and calculates the total weight of each combination. The calculation control unit 4 also compares the total weight value of each combination pattern with the set value Wa from the target weight setting unit 6, and finds the combination, namely the aforementioned optimum combination, the total weight value of which is equal to Wa or closest to Wa within the upper and lower limit weight values Wp, Wo from the upper and lower limit weight value setting units 5, 7, respectively. The calculation control unit 4 applies a discharge signal Se to the weighing hoppers of the weighing machines corresponding to the selected optimum combination, causing these weighing hoppers to discharge their articles. The calculation control unit 4 applies a supply signal Sf to those pool hoppers corresponding to the weighing machines that have discharged their articles, whereby these pool hoppers dump their articles into the underlying weighing hoppers of said weighing machines. It should be noted that the lower limit weight value setting unit 7 may be deleted from the illustrated arrangement so that the articles may be discharged from the weighing machines using only the upper limit value and the set target weight. This method is preferred when weighing out commodities.
There are occasions when an optimum combination cannot be obtained. The usual practice at such time is to supply prescribed weighing machines with additional articles in order to raise the precision of the combinatorial weighing operation. This supplementary feed of articles is carried out for all weighing machines that provide weight data indicative of a weight value less than a fixed value, or for a prescribed number of weighing machines whose weight data indicate weight values small in comparison with the other weight values. Conventionally, the changeover between these two supplementary feed modes is performed by an operator who, relying upon his experience, operates a changeover switch to select the particular mode. This constitutes a disadvantage of the prior art since the necessity of the changeover switch results in a combinatorial weighing apparatus of a more complicated construction and operation.
A combinatorial weighing apparatus operates on the premise that the amount of articles supplied to it from a commodity production system located upstream is approximately constant. It is therefore required that the weighing apparatus have an article processing capability slightly higher than that actually needed to process the articles with which it is supplied. In order to assure that the weighing apparatus will have such a capability, two configurations of the weighing apparatus are available. One is a high-speed, low-capacity combinatorial weighing apparatus for dealing with a target weight value that is comparatively small. The apparatus executes so-called high-speed, low-capacity processing and performs combinatorial weighing at a high rate of speed. The other is a low-speed, high-capacity combinatorial weighing apparatus for dealing with a target weight value that is comparatively large. The apparatus executes so-called low-speed, high-capacity processing and performs combinatorial weighing at low speed. The two types of apparatus differ in terms of the number of weighing machines and the capacity of the weighing hoppers for suitably adjusting the amount of articles supplied per weighing machine by distributive feed.
In a case where the production schedule requires that the high-speed, low-capacity combinatorial weighing apparatus execute low-speed, high-capacity processing, the amount of articles supplied to each weighing machine will be too small if the target weight value is merely raised. This means that a large number of the weighing machines will be selected for the optimum combination, which makes it necessary for all of the weighing machines to participate in the combinatorial calculations for selecting the optimum combination. This prolongs the time needed for the calculations and necessitates modification of the calculation program.
One method of operating the combinatorial weighing apparatus is to divide a single target weight value in half, perform combinatorial weighing twice,, i.e., in two cycles, once for each half of the target value, and discharge the articles at each of the two cycles to arrive at a total weight value which is equivalent to the target weight value. This is so-called multiple weighing processing. Nevertheless, a combinatorial weighing apparatus for practicing this method still necessitates modification of the combinatorial calculation program as well as a modification of circuitry for allowing the packing machine, which is connected to the weighing apparatus, to perform a single packing and sealing operation only after receiving the discharge signal from the weighing apparatus twice. Such an arrangement is inconvenient for dealing with unexpected or sudden changes in the production schedule.
Though it is possible to adopt the low-speed, high-capacity type combinatorial weighing apparatus in which the amount of articles supplied to each weighing machine is increased and a batch of the articles is discharged through a single combinatorial calculation, adjustment of the amount of articles supplied is troublesome, a considerable number of unsatisfactory weighing cycles occur until supply is stabilized, and the capacity of the weighing hoppers imposes a limitation upon the amount by which the supply of articles can be increased.